We were just starting to get a bit of sea ice before the temperatures shifted and we now seem to be back to strangely spring-like conditions. Nonetheless, I was scheduled to teach a class at my daughter’s school on sea ice, so it has been on my mind. Sadly, I was not able to bring samples of sea ice in. But, fortunately, I’ve done a bit of kitchen experimenting in the past and was able to do an experiment with homemade salt ice. Ocean water has about 35 grams of salt per 1,000 grams of water, so you can mix this up pretty easily. It’s a good way to learn a lot about ice.
Ice formation is a strange thing. It all has to do with energy. When water molecules slow down enough, they form crystals creating a solid rather than a liquid. This requires two things – that the water is still enough and that it is cold enough. Given that many bodies of water are constantly in motion, it may have to be colder than usual for a vigorously moving body or water to freeze. But, in general, freshwater freezes at 32 F. Salt water doesn’t freeze until the temperature falls to 29 F.
That’s why we put salt on our driveways to melt the ice. What the salt does is to require that the temperature be colder for the water to stay frozen. It does this bit by bit by mixing with a layer of water on the surface. Once that is melted, it begins to work its way into the next layer – hopefully eventually making it to the pavement.
But there’s something else that’s different about the way salt ice forms. When salt ice develops along the coast, it can create strange forms as it breaks apart and piles up. It can form whimsical caves amidst the rocks that make for rare seasonal seaside explorations. One winter when we had particularly fantastic coastal ice formations, tired out from clambering over and through them, one of my daughters was desperately thirsty and decided to sample the ice. “It’s not salty,” she said. In disbelief, as her know-it-all marine scientist mother, I said something like, “you must just be really thirsty.” But, she is very insistent and eventually got me to try it – and it turns out she was right.
This seemed really strange to me. A bit of research helped me to understand that, as the water freezes, the water molecules stick together in crystals that push the salt aside. That leaves ice that is mostly fresh. A kitchen experiment that shows this is to make salt ice cubes and taste the top of them. It’s refreshingly fresh. Taste the bottom of the cube and you’ll find it quite salty. Saltwater is denser, so the salty part sinks to the bottom. There’s one other difference. You may also notice that the top of the cube is clear and the bottom is cloudy. The ice crystals pack more densely than the salt crystals, making a clear versus a cloudy ice.
One more difference – the salty party is also mushy. This may be familiar if you’ve tried to walk on salt ice and realized it was more like slush than a hard surface. It’s a little disconcerting even if you know it is plenty thick. The salty parts of the ice have a funny texture with lots of pockets of trapped air. You can see this in large form when looking at slabs of sea ice. The tops of the slabs are shiny and slick whereas the bottoms flake off into handfuls of salt crystals. It’s a magical thing to brush them off into the sunlight on a clear winter day.
One final note about salt ice or any type of ice for that matter is its importance as an insulator. Ice, like snow, acts as a giant blanket that covers all kinds of creatures living beneath it. It keeps the water temperature much warmer than it would otherwise be if exposed to the air, helping to protect a functioning, if slowed, ecosystem. Salt ice might not be quite as good for drumming as the clear lake ice in Russian that I mentioned last week, but it is complicated stuff and worth some exploration as well as a bit of kitchen science.
Comments are not available on this story.
Send questions/comments to the editors.
